linux-kernel - Re: [PATCH] arm64: armv8_deprecated: Fix undef

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Date:   Tue, 7 Apr 2020 10:27:45 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Fredrik Strupe <fredrik@...upe.net>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] arm64: armv8_deprecated: Fix undef_hook mask for thumb
 setend

On Mon, Apr 06, 2020 at 04:16:05PM +0200, Fredrik Strupe wrote:
> Use a full 32-bit mask to prevent accidental matchings of thumb32
> instructions where the second half-word is equal to the thumb16 setend
> encoding.
> 
> This fixes the same problem as the following patch:
> 
>     https://lkml.org/lkml/2020/3/16/341

This link is not guaranteed to be stable and the commit should have the
full description rather than referring to another email.

> but for setend emulation instead.
> 
> Signed-off-by: Fredrik Strupe <fredrik@...upe.net>

It also needs Fixes: and Cc: stable tags.

> ---
>  arch/arm64/kernel/armv8_deprecated.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
> index 9d3442d62..8c06dfee0 100644
> --- a/arch/arm64/kernel/armv8_deprecated.c
> +++ b/arch/arm64/kernel/armv8_deprecated.c
> @@ -609,7 +609,7 @@ static struct undef_hook setend_hooks[] = {
>  	},
>  	{
>  		/* Thumb mode */
> -		.instr_mask	= 0x0000fff7,
> +		.instr_mask	= 0xfffffff7,
>  		.instr_val	= 0x0000b650,

I can see how this could happen but it would be useful to provide a
concrete example in the commit log.

The instruction opcode built by call_undef_hook() first reads a u16 as a
T16 instruction and the above should be fine. However, if this looks
like a T32 opcode, it reads a subsequent u16 which becomes the lowest
half-word and the above mask/val may inadvertently match it.

The patch looks fine to me as long as you update the commit log.

Thanks.

-- 
Catalin